netdev - Re: [PATCH v3] tcp: add support for SO_PEEK

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Message-ID: <CANn89i+bc=OqkwpHy0F_FDSKCM7Hxr7p2hvxd3Fg7Z+TriPNTA@mail.gmail.com>
Date: Tue, 13 Feb 2024 20:31:34 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: kuba@...nel.org, passt-dev@...st.top, sbrivio@...hat.com, 
	lvivier@...hat.com, dgibson@...hat.com, jmaloy@...hat.com, 
	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH v3] tcp: add support for SO_PEEK_OFF

On Tue, Feb 13, 2024 at 7:39 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Tue, 2024-02-13 at 16:49 +0100, Eric Dumazet wrote:
> > On Tue, Feb 13, 2024 at 4:28 PM Paolo Abeni <pabeni@...hat.com> wrote:
> > > On Tue, 2024-02-13 at 14:34 +0100, Eric Dumazet wrote:
> > > > This sk_peek_offset protocol, needing  sk_peek_offset_bwd() in the non
> > > > MSG_PEEK case is very strange IMO.
> > > >
> > > > Ideally, we should read/write over sk_peek_offset only when MSG_PEEK
> > > > is used by the caller.
> > > >
> > > > That would only touch non fast paths.
> > > >
> > > > Since the API is mono-threaded anyway, the caller should not rely on
> > > > the fact that normal recvmsg() call
> > > > would 'consume' sk_peek_offset.
> > >
> > > Storing in sk_peek_seq the tcp next sequence number to be peeked should
> > > avoid changes in the non MSG_PEEK cases.
> > >
> > > AFAICS that would need a new get_peek_off() sock_op and a bit somewhere
> > > (in sk_flags?) to discriminate when sk_peek_seq is actually set. Would
> > > that be acceptable?
> >
> > We could have a parallel SO_PEEK_OFFSET option, reusing the same socket field.
> >
> > The new semantic would be : Supported by TCP (so far), and tcp
> > recvmsg() only reads/writes this field when MSG_PEEK is used.
> > Applications would have to clear the values themselves.
>
> I feel like there is some misunderstanding, or at least I can't follow.
> Let me be more verbose, to try to clarify my reasoning.
>
> Two consecutive recvmsg(MSG_PEEK) calls for TCP after SO_PEEK_OFF will
> return adjacent data. AFAICS this is the same semantic currently
> implemented by UDP and unix sockets.
>
> Currently 'sk_peek_off' maintains the next offset to be peeked into the
> current receive queue. To implement the above behaviour, tcp_recvmsg()
> has to update 'sk_peek_off' after MSG_PEEK, to move the offset to the
> next data, and after a plain read, to account for the data removed from
> the receive queue.
>
> I proposed to let introduce a tcp-specific set_peek_off doing something
> alike:
>
>         WRTIE_ONCE(sk->sk_peek_off, tcp_sk(sk)->copied_seq + val);
>
> so that the recvmsg will need to update sk_peek_off only for MSG_PEEK,
> while retaining the semantic described above.
>
> To keep the userspace interface unchanged that will need a paired
> tcp_get_peek_off(), so that getsockopt(SO_PEEK_OFF) could return to the
> user a plain offset. An additional bit flag will be needed to store the
> information "the user-space enabled peek with offset".
>
> I don't understand how a setsockopt(PEEK_OFFSET) variant would help
> avoiding touching sk->sk_peek_offset?
>

I was trying to avoid using an extra storage, I was not trying to
implement the alternative myself :0)

If the recvmsg( MSG_PEEK) is supposed to auto-advance the peek_offset,
we probably need more than a mere 32bit field.


> Thanks!
>
> Paolo
>